Sensors measure environmental parameters such as luminosity, humidity, wind speed, and the presence of liquids at different points in time and space. For example, fluid levels, fluid temperatures, the presence of a liquid, fluid viscosity, and fluid velocity are all measurable parameters.
Detection technology relies on sensors to interact and measure our world. These sensors, when part of a probe, detect the evolution of any of a large plurality of environmental parameters. Some sensors are programmed to measure continuously the parameter they are following, while other sensors are programmed to set off using, for example, a relay, once a programmed cut-off value is reached. The current disclosure is mostly directed at set-off detectors with cut-off values.
Many types of sensor-based probes exist, including, for example, mechanical sensors, floats, electromechanical sensors, radars, visual sensors, weight sensors, laser sensors, ultrasonic sensors, fluid resistance/conductivity sensors, and even capacitance sensors. While the described technology applies equally to each of these technologies, what is contemplated and disclosed as a first embodiment is the use of the technology adapted for the measure of the resistance or the conductance of fluids, and more precisely as shown in one embodiment, a measure of the conductivity between two conductive probes spaced at a short distance in a fluid to determine the resistance of the volume of fluid between the two probes.
Probes without automatic sensitivity settings are known in the art. Lumenite Control Technology, Inc., the assignee of the current invention, markets a system without automatic sensitivity setting as a microprocessor-based pasteurization testing system shown as prior art as FIG. 1. The MTC-2000 as shown on FIG. 1 measures the velocity of milk in a heated tube as required by the FDA in the milk pasteurization process. The U.S. Public Health Code requires pasteurization of milk to be conducted at a temperature of at least 160° F. at a minimum duration of 15 seconds. In operating conditions, heated milk runs along a heated serpentine tube shown at the bottom of FIG. 1. The control of the fluid velocity allows for a determination of the time of passage by a reverse calculation from the time needed to travel the distance between both probes along the line.
Since water has a viscosity and thermal inertia close to that of milk, testing can be conducted using water instead of milk. Two probes are placed in series along the holding tube. The resistance-based probes are designed to prevent the accumulation and growth of bacteria and fungi where the probes are connected using a hex nut. U.S. Pat. No. 2,470,066 to G. V. Calabrese describes an electrode assembly with different probes having hex nuts that may be used during the pasteurization process. This patent is hereby incorporated by reference.
The probes are made of two partly insulated conductive rods placed side-by-side to measure the electrical resistance value of the fluid located between the rods. The use of two rods allows for measurement of two elements placed at different voltages. Once the flow of water is established, a baseline resistance is measured, a value generally within the range of 500Ω to 200,000Ω, or a range of 100Ω to 50,000Ω based on the device. Ranges are variable and depend upon the different parameters of the detection device. Using a variable resistance set-off dial, a user sets a fixed set-off value based on the estimated value of the resistance of the water in the system before measurement using the MTC-2000 begins.
To measure the time needed for fluid to travel from the first probe to the second probe, a liquid having a different electrical conductivity is introduced in the line before the first probe. A MTI-1000 liquid injection apparatus, also from Lumenite Control Technology, Inc., can be used to introduce a saline solution. When the solution reaches the first probe, the resistance drops sharply below the set-off value of the first probe to trigger the relay. The microprocessor of the MTC-2000 then measures the time until a similar drop at the second probe occurs. If the set-off value trigger point is not far enough from the baseline resistance of the probe, false positives can be measured as the saline water mixes with water.
In addition, the actual value of the resistance can be offset by a plurality of factors. For example, if milk has formed a dried layer over the probe body, the measured resistance increases significantly. If the water source contains more electrolytes, the measured resistance drops.
One of the major drawbacks of this system is the inherent variability of the resistance and conductivity measurements between the two rods in the probes. While the conductivity or resistance of the water must be initially measured to obtain a baseline, this value can oscillate within a margin of error and have other small variations. Once the average value of the water resistance is determined, the operator must then fix a detection level that signals the arrival at the probes of a fluid having higher conductivity. For example, if a resistance of 10,000Ω is measured for the water, then this value must be calibrated. The detection level of the saline solution must also be entered, for example, at 5,000Ω. What is needed is an autosensitive probe and system capable of automatic adjustment that does not require user estimations.